The present invention relates to microdischarge devices and, in particular, new structures for light emitting devices and low-cost methods of producing ultraviolet or visible light from thin sheets.
It has long been known that electrical discharges are efficient sources of light and, today, gas discharge lamps (including fluorescent sources, and metal-halide, sodium, or mercury arc lamps) account for most of the world's light-generating capacity (several billion watts on a continuous basis). Most of these devices are, unfortunately, bulky and frequently have fragile quartz or glass envelopes and require expensive mounting fixtures. In addition to general lighting, discharges produce ultraviolet and visible light for other purposes such as germicidal applications (disinfecting surfaces and tissue), cleaning electronic and optical surfaces in manufacturing, and activating light-sensitive molecules for medical treatments and diagnostics.
Although microdischarges were demonstrated by A. D. White in 1959, only recently were microdischarge devices fabricated in silicon by techniques developed in the integrated-circuit industry. As described in U.S. Pat. No. 6,016,027, the first microdischarge devices made in silicon had a cylindrical microcavity that served as the cathode of the device. The semiconductor cathode was affixed to a copper heat sink with conductive epoxy. The anode for the microdischarge device was typically a metal film such as Ni/Cr. A thin dielectric layer deposited onto the silicon electrically insulates the cathode from the anode. When the microcavity is filled with the desired gas and the appropriate voltage imposed between the anode and cathode, a discharge is ignited in the microcavity.
Microdischarges have several distinct advantages over conventional discharges. Since the diameter of single cylindrical microdischarge devices, for example, is typically less than 400-500 μm, each device offers the spatial resolution that is desirable for a pixel in a display. Also, the small physical dimensions of microdischarges allows them to operate at pressures much higher than those accessible to conventional, macroscopic discharges. When the diameter of a cylindrical microdischarge device is, for example, on the order of 200-300 μm or less, the device will operate at pressures as high as atmospheric pressure and beyond. In contrast, standard fluorescent lamps, for example, operate at pressures typically less than 1% of atmospheric pressure.
Despite their applications in several areas, including optoelectronics and sensors, silicon microdischarge devices have several drawbacks. For example, the annular metal anodes used in early microdischarge devices have short lifetimes because of sputtering. After operating for as little as several hours, damage to the anode is visible and devices frequently fail after only tens of hours of operation. Optical emission from metal atoms evaporated from the anode is easily detected prior to failure of the device. One solution is to replace the metals tested to date with a more robust material, such as polycrystalline silicon or tungsten. However, these materials increase the fabrication cost and difficulty, do not yield significantly increased output power and may not yield significantly improved device lifetime.
Furthermore, silicon is brittle, comparatively high in cost, and single wafers are limited in size (12″ in diameter currently). In addition, silicon fabrication techniques, although well-established, are labor and time intensive and, therefore, not suitable for low-cost applications. Therefore, a number of potential applications of microdischarge devices, not presently accessible with silicon (or other) semiconductor technology, could be pursued if low-cost, flexible microdischarge arrays, requiring voltages no higher than that available in common wall sockets, were available.
Two other drawbacks of previous microdischarge devices and arrays concern the inefficiency of extracting optical power from deep cylindrical cavities and the difficulty in scaling the size of arrays. If the cylindrical cathode for a microdischarge is too deep, it will be difficult for photons produced below the surface of the cathode to escape. Another problem arises in fabricating arrays of microdischarge devices is that devices at the perimeter of the array ignite preferentially and arrays as small as 10×10 are difficult to ignite at all.